The potential utility of ribonucleoside analogues as antiviral agents arises principally from the fact that genetic transcription and replication of viral genomes almost always depends on the activity of a new, virus-specific enzyme at some stage of the intracellular life cycle. We have characterized two RNA polymerizing activities in the Reovirus infected cells. One which transcribes the ds genome and synthesizes functional messengers. The other uses the plus strand as template and synthesizes exclusively minus strands. By comparing the cellular DNA-dependent RNA polymerase with these two activities in vitro and in vivo will make it possible to explore the mechanism of action of prospective drugs on both cellular and on all viral polymerizing functions. The role of oligo A in the replicative cycle of Reovirus will be explored. Special attention will be focused on the initiation process of double-stranded genomes.